1. Field of the Invention
The invention relates to a method and apparatus for the micromachining of aerofoil surfaces, particularly the turbine blades and vanes of jet engines and other high precision turbines.
2. Description of the Related Art
A turbine blade or vane of a modern jet engine has two distinct parts: an anchorage portion and an aerofoil blade portion. The anchorage portion is ground to create one or more flat planar locating faces which securely and precisely anchor the blade or vane in the engine. The blade portion is cast, and therefore has a surface which is finished to a lower standard of accuracy than the ground faces of the anchorage portion.
Gas turbines generally, and jet engines in particular, run at very high internal temperatures, often higher than the melting point of the alloy used for the turbine blades or vanes. The blades or vanes are therefore protected from melting by creating a film of cooling air over all of the aerofoil surfaces that would otherwise be exposed to the hot combustion gases. The cooling air is discharged from a plenum chamber internally of the blade or vane, and out through an array of small and accurately aligned and located holes in the aerofoil surface. Those holes are typically micromachined using a laser mounted with five degrees of movement relative to the aerofoil surface of the blade or vane.
A problem associated with the micromachining of the air holes in the aerofoil surface is the need accurately to position the holes relative to the aerofoil surface itself rather than relative to the mounting. The aerofoil is typically mounted in the laser cutting machine by its machined mounting portion. Although that mounting portion is a high precision ground sub-element of the component, it may not be precisely aligned relative to the shaped aerofoil surface. Two alternative methods have been proposed for ensuring that the laser-drilled holes are accurately positioned relative to the aerofoil surface rather than relative to the mounting portion. Each method requires the use of a separate piece of setting or measuring equipment in the machine workshop, and each has its own associated disadvantages.
In a first method, in a setting apparatus the component is initially positioned in a compliant mounting which engages with the ground-flat faces of the anchorage portion of the component. The compliance of the compliant mounting is such as to provide the aerofoil surface of the component with at least three, and preferably five, degrees of movement in space. The compliant mounting is itself held in the setting apparatus, and jaws and probes of the setting apparatus are moved into abutment with the aerofoil surface of the component, to grip the aerofoil surface and hold the component by its aerofoil surface in a precise orientation in space. At that stage the compliant mounting is tightened and locked solid, prior to its release from the setting apparatus, so that when the assembly of component and compliant mounting is carried over to the laser drilling machine, the component may be held by anchoring the compliant mounting against datum surfaces of the laser drilling machine. Drilling can start immediately, because the aerofoil surface will be correctly aligned relative to those datum surfaces. Apart from the need for a separate piece of setting apparatus in the machine workshop, for the positioning of the component relative to the compliant mounting prior to tightening that compliant mounting, this method of component alignment suffers from the disadvantage that if the component is mishandled while being transported from the setting apparatus to the laser cutting machine, the component may move in its mounting so that the accuracy of alignment can be lost.
In a second method for ensuring the accuracy of the laser-drilled holes relative to the aerofoil surface, the component is first mounted in a measuring apparatus which engages directly the ground faces of the anchorage portion of the component. Probes or sensors of the measuring apparatus then map the position and shape of the aerofoil surface of the component relative to the ground faces of the anchorage portion, and from the output of those probes or sensors a laser drilling control program is compiled, for the control of the laser tool relative to the aerofoil surface in the subsequent micromachining step. The compiled laser control program and the component are then removed from the test rig and kept together until the micromachining is to take place. Then the component is mounted in the laser cutting machine, the control program loaded into the laser cutting machine, and the micromachining of the air holes takes place with accurate positioning of those air holes relative to the aerofoil surface. This method also requires a separate piece of workshop apparatus, and has the additional disadvantage that if the compiled laser control program becomes separated from the component prior to machining, then the component must be returned to the measuring apparatus for the re-measurement of the aerofoil surface position and the compilation of a new program. A potentially much greater disadvantage of this method is that if two components and their two accompanying compiled programs are interchanged accidentally between the measuring apparatus and the laser machine, then the laser cutting can take place with the holes being bored in each component using the program compiled for the other component. The exchange of program information may not be apparent from a visual inspection of the finished components, but the holes will be offset or misaligned from their optimum positions relative to the respective aerofoil surfaces, and the components will be liable to early failure in the extremely demanding conditions experienced in use.
The invention has as its object the avoidance of the above problems and the establishment of a method of micromachining holes in aerofoil components in which the holes are machined accurately and consistently relative to the aerofoil surfaces and in a manner more economical than previously.
The invention provides a method for micromachining the aerofoil surfaces of a component which comprises a ground anchorage portion and a cast aerofoil portion, which method comprises:
mounting the component in a laser machining apparatus by means of its anchorage portion, the laser machining apparatus being preprogrammed to machine holes in the aerofoil surface when that surface is in a predefined reference position and orientation in the apparatus;
moving a probe or probes over the surface of the aerofoil surface of the cast aerofoil portion to derive the positions of selected points on that aerofoil portion;
calculating from the derived position data angular and linear offset data to define the deviation of that cast aerofoil portion from the reference position and orientation defined in the computer memory of the laser machining apparatus; and
applying that angular and linear offset data to the computer program resident in the laser machining apparatus to control the movement of the laser head relative to the aerofoil surface and to machine the air holes in the aerofoil surface.
The method of the invention avoids all potential for misalignment of the laser head relative to a previously measured or set position of the aerofoil surfaces of the component being machined, because the component is never removed from one mounting and placed in another during the course of the method. The component remains in the laser machine from the beginning of the probe test run to establish the position and orientation of the aerofoil surfaces to the end of the machining process. Moreover only a single set of angular and linear offset data is created and stored according to the invention, and that data is applied to the program resident in the laser machine to control, in real time, the movement of the laser head.
Since the laser beam is likely to be a parallel focussed beam of circular section, only five pieces of offset data need to be derived: linear offsets along the x, y and z axes and angular offsets about the a and b axes.
The probe or probes may be mounted on the movable platform on which the laser head is mounted, or on a similar platform, so that essentially the same motors and/or software can be used for positioning the probe or probes relative to the component as will subsequently be used for aligning the laser head relative to the component.
Any suitable algorithm may be used for calculating from appropriate derived position data the angular and linear offsets of the aerofoil surface of the component. Preferably the derived position data are obtained from six probe measurements: four being the positions of two pairs of associated tangential points at which an outer curved edge of the aerofoil surface touches tangentially a linear probe; and two being the positions of points at which mutually perpendicularly aligned probes touch the aerofoil surfaces of the component.